Attenuation of radar energy impinging on a surface can be achieved by destructive interference. A radar wave reflected from a surface has a maximum electric field at one-quarter of its wavelength from the reflective surface. A resistive material placed at one-quarter wavelength from the reflective surface conducts current and reduces the energy of the maximum electric field, thereby attenuating the reflected radar wave. An example of this type of attenuator is the Salisbury screen wherein a thin layer of controlled conductivity (often known as spacecloth) is spaced from a reflective surface such as a metal sheet at a distance equal to one-quarter of the wavelength of the radar to be attenuated. The spacecloth conventionally has an impedance of approximately 377 ohms per square which is the characteristic impedance of free space. By having the impedance of the spacecloth substantially the same as that of free space no substantial reflection occurs therefrom.
As an improvement on the Salisbury screen the same principle is employed wherein a plurality of impedance layers having controlled electrical properties are spaced successively from a metal reflective surface with each of the sheets being a distance corresponding to one-quarter wavelength of radiation of a particular frequency. Since the attenuation of a single sheet interference absorber is actually over a narrow band rather than sharply at a specific frequency, and because of interaction between the successive layers in a multiple layer interference attenuator, there can be substantially continuous attenuation of radar over a relatively broad frequency range.
In order to obtain good attenuation over a broad frequency range it is necessary to carefully control the spacing between successive layers, the dielectric properties of the spacing material, and also the characteristic impedance of each of the layers. When a plurality of impedance layers are employed substantial quality control problems are encountered in achieving the desired impedance values in all of the attenuator layers. A typical interference type absorber as provided in the prior art is described and claimed in copending U.S. patent application Ser. No. 305,564 entitled, "Multilayer Structure" by L. J. Costanza et al, and assigned to North American Rockwell Corporation, the assignee of this application.
Control of the electrical properties of attenuator layers at nominal temperatures is severe enough. However, the problem is particularly acute at elevated temperatures where the materials available for producing a good interference type attenuator are limited in number and are difficult to handle.
It is therefore desirable to produce a radar attenuator wherein the requirement for precise control of impedance of a plurality of layers is minimized or eliminated.